Just a quick question on hot water cylinders.
We have a large cylinder (with larger heat exchange coils due to it
being fed by a geothermal source .. lower working temp than oil/gas).

The heat pump guy made a brief visit and was a bit puzzled that the
pipe going in at the bottom of cylinder was hotter than at top. He
told me to query my plumber as to where the flow and return had
been plumbed. He mentioned that it is normal to have flow (i think)
plumbed into top of cylinder (hotter) and return on bottom.

Does this make sense? I just wanted my facts right before consulting
the plumber responsible for the cylinder install/plumbing.

In an unpressurized system when you turn on a hot tap the pressure from the tank in the attic forces cold water into the bottom of the cylinder, this forces the hotter water out the pipes at the top of the cylinder which are connected to your taps/shower etc.

Though after rereading your post I think you are asking about the flow return for the heat exchanger?

Not knowing anything about it ( ) Im woudl guess that the bigger the difference between the two temperatures the more heat that is exchanged.
If you had the flow from the exchanger at the top of the cylinder then the water in cylinder could end up heating the water in the heat exchanger!
The warm water in the heat exchanger passes through the coldest water in the cylinder and the heat is exchanged from one to the other.

Just a quick question on hot water cylinders.
We have a large cylinder (with larger heat exchange coils due to it
being fed by a geothermal source .. lower working temp than oil/gas).

The heat pump guy made a brief visit and was a bit puzzled that the
pipe going in at the bottom of cylinder was hotter than at top. He
told me to query my plumber as to where the flow and return had
been plumbed. He mentioned that it is normal to have flow (i think)
plumbed into top of cylinder (hotter) and return on bottom.

Does this make sense? I just wanted my facts right before consulting
the plumber responsible for the cylinder install/plumbing.

An alternative view if I may with no intention to insult anyone

As we all know, hot water rises and cold water falls.

In a normally set up heat exchanger [HE] the hot water enters from the top and as it cools in the HE it gets heavier and will tend to fall through the HE and pull the water behind it, hence tending to increase the flow through the HE. A siphon effect if u wish.
Heat transfer is a function of many things, the one in question here is transit time through the HE.

The reverse will increase the transit time in the HE and may increase the efficiency of the HE in this particular context.

In a normally set up heat exchanger [HE] the hot water enters from the top and as it cools in the HE it gets heavier and will tend to fall through the HE and pull the water behind it, hence tending to increase the flow through the HE. A siphon effect if u wish.
Heat transfer is a function of many things, the one in question here is transit time through the HE.

The reverse will increase the transit time in the HE and may increase the efficiency of the HE in this particular context.

Interesting, Ircoha ...

So, in effect you are saying that there might be a method in the apparent madness of plumbing the flow and return from/to heat pump in the unconventional way (flow/top cylinder, return/bottom) ?
I went into the attic this evening and indeed the hotter pipe is the
one nearest the bottom. I was able to identify the other inlet/outlet
pipes (one coming from the main water tank and one feeding the
hot water taps in the house...i assume (at the very top of cylinder),
It's an indirect cylinder which is very well insulated.

As an aside.. I spoke to a knowledgeable engineer about the underfloor
controls (my heat pump was cycling and we weren't getting too much
heat late in the evenings). Our timer/controller unit which controls
priority of domestic hot water over underfloor delivery was supplied
by the UFH guys rather than the heat pump folks but it is very reputable.
The thing is ( I believe...) I am going to extend the hours that I have
the heat pump being called for heat during the offpeak period (12am-6am)
and I think this will give enough time for both hot water to be heated
as well as putting store into the downstairs floors so that they might
just need a topup (on a chilly evening). I've set the room stats to
20c in the times that they are calling for heat and 15C as the lowest
room temp (so that at daytime at the moment they will hardly ever
call for heat unless the room temp dips very low).

Circulation is badly affected ,depending on the type of circuit.
Grafity circuit only works by falling down from flow to return.

The water will take longer to heat if it's done wrong. The water that enters at the bottom cools before it gets to the top, this makes it harder for the water to heat (heat rises).
On a lower temperature system ,I imagine it would take a long time to reach any sort of temperature.